This invention relates to a separating device for trimming the width of a poured curtain of coating material in a coating device, and more particularly prior to the curtain's impinging on a substrate, in particular a printed circuit board, to be coated which substrate is passed transverse to the direction of curtain flow underneath a foot end of the separating device at a distance from the latter while the separating device is inclined, relative to the perpendicular, at an angle such that the upper end of the separating device projects beyond the foot end of the same in a direction toward the central region of the curtain and projects separatingly into a lateral region of the curtain, so that a marginal strip of the curtain is separated from the central region thereof and the coating material of that strip is diverted laterally, while the trimmed central region of the poured curtain is enlarged downwardly being spread laterally toward the separating device.
Severing organs of this kind have been disclosed more recently for use in trimming devices of coating apparatus described, for instance, in U.S. Pat. No. 4,559,896 to Bossard et al and in published European patent application Ser. No. 145,648. The severing organs described therein of which usually one is provided for trimming the left-hand margin and another one for trimming the right-hand margin of a poured curtain of coating material comprise each a first, upper separating element which projects with its upper end into the poured curtain and trims the width of a curtain while having a guiding edge at its lower end and simultaneously leading off laterally the coating material of the severed marginal curtain region, as will as a second separating element below the first one and being laterally, slightly inwardly staggered, which second separating element narrows the width of the poured curtain further by trimming a marginal zone thereof which could lead to the formation of an intermediary rim bead. This lateral curtain edge guide can comprise a lower segment which is either inclined downwardly inwardly, i.e. toward the middle region of the curtain whereby a reduction of the width of this curtain middle region will be effected, or it is inclined downwardly outwardly, therefore achieving a slight widening or "spreading" of the curtain accompanied by a corresponding reduction of its thickness. Moreover, the lateral guidance of the marginal region of the trimmed curtain middle region can comprise an upper segment which is inclined outwardly in downward direction, and, associated with that segment in downward direction, a second slightly inwardly inclined segment.
The outwardly inclined upper segment of this known severing organ can enclose, with the perpendicular, an angle .beta. of preferably up to 5.degree. while a segment provided downwardly of the first segment and being inwardly inclined preferably encloses, with the perpendicular, an angle .alpha. from 1 to 8.degree. which is shown to be considerably larger than the angle .beta. as shown in FIG. 4 of U.S. Pat. No. 4,559,896.
Trimming of a curtain of coating material and in particular of the type of coating material which is used in the coating of printed circuit plates, is required because of the formation of marginal beads which occurs as the curtain descends and is being deposited onto the moving substrate and carried along on the latter. Such bead formation does not occur with any comparable disturbing effect if the descending curtain consists of water. Devices for controlling the width of a water curtain have been described in French patent application Ser. No. 85,05634 by IRSID, published as No. 2,580,199 on Oct. 17, 1986.
The particular phenomenon occurring when coating materials for the above purpose are involved is noticeable already in the descending curtain not having a constant curtain thickness across its entire width. Along the lateral curtain rim guiding elements which are always required a pronounced marginal bead is unavoidably formed which extends by a shorter or longer distance into the central curtain region depending on the rheological properties of the coating material. The width of this marginal bead zone amounts on each of the two longitudinal curtain rims and correspondingly along the longitudinal rims of the coating zone or strip being deposited on the substrate being coated, to about 5 to 20 mm. The height of the marginal bead protruding above the deposited coating can be as much as 1 to 2 mm. When manufacturing electronic circuit plates, e.g. for electronic apparatus which require circuit plates in which a marginal zone about the plate top or bottom face is to remain uncoated and which are to contain numerous extremely thin conducting lines, the formation of such beads, for instance, when a coating is applied as solder-stopping shields or masks, is most disturbing or even completely inadmissible. Further processing of such plates having marginal beads in their coating or coated regions can be made very difficult.
In U.S. Pat. No. 4,060,649 to Coleman, guide plates have been described which are provided at either edge of a curtain of paint being applied to a substrate. It had been noted that the resulting paint coatings were uneven because the curtain tended to draw together or at least to wrinkle. The inner edges of the guide plates to which the paint curtain edges cling by capillarity must be sloped inwardly, as had been suggested for the lowermost guiding segment shown, for instance, in FIGS. 3 and 4 of U.S. Pat. No. 4,559,896, supra.
It has been found that guiding elements of the type described in U.S. Pat. Nos. 4,060,649 and 4,559,896 do not suppres the undesirable formation of marginal beads or reduce the height and width of the bead zones only to an unsatisfactory degree.
To the extent that such reduction is achieved especially by some of the trimming devices described in U.S. Pat. No. 4,559,896, the achieved reduction of the size of the marginal beads has been rendered insufficient by the rapid advances in the field of printed circuit plate technology. For, it is nowadays no longer sufficient to keep the marginal zones at the opposite longitudinal edges of the plates free from being coated by the poured curtain impinging on the plates as they advance through the coating apparatus at substantially even speed, but the coating being applied, in particular, as a solder-stopping mask is often to cover only certain selected areas of the circuit plate; and in this case marginal beads about such areas are particularly undesirable.
Thus it had been found that coatings produced with the coating apparatus of U.S. Pat. No. 4,559,896 (or the corresponding EP-A-0 145,648) still contain marginal beads which, when the thickness of the applied coating layer after drying is about 30 micrometers (mcm), will extend over a marginal zone having a width of about 10 to 20 mm, which corresponds to a reduction of the width of the usable circuit plate area by about 20 to 40 mm, taking both opposite marginal zones into account; and this does not take into account losses of usable area occurring in the internal area of the circuit plate when only parts of that area are to be covered by solder-stopping masks. The marginal beads often occurred as double beads of which the outer bead can attain up to 110 mcm and the inner bead a height of 70 mcm when the coating is to serve as solderstopping mask was applied by the coating process using the solution described in column 3, line 62 till column 4, line 7, of U.S. Pat. No. 4,230,793 (corresponding to European patent 2040), when a curtain of the solution having a thickness of about 40 mcm is poured with a speed of 90 meters per minute onto a printed circuit plate moved transversely thereto at approximately the same speed, thereby producing a coating having undried a weight of about 7.5 g per 600 cm.sup.2 of circuit area. The thickness of the coating after drying, applied under the abovedescribed conditions and using the above-mentioned solution as coating material, amounts to about 25 to 35, and preferably about 28 to 30 mcm.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore a first object of the invention to provide a separating device of the initially-described type which considerably simplifies the relatively lavish, somewhat more expensive construction requiring two separating elements in each of the two severing organs used in the known device of U.S. Pat. No. 4,559,896, and which nevertheless affords substantially narrower, less high beads than are produced with the last-mentioned known severing organs.
It is another object of the invention to provide means for maintaining a purely laminary flow of the coating material not only in the middle curtain zone but also in the laterally spread, extended side region or regions E thereof and thus to contribute to keeping any marginal beads that might still occur narrower and lower than it is possible to achieve with the known severing organs.
These objects and others that will be apparent from the following description are attained in accordance with the invention by a separating device of the initially-described type in which the device consists of a single separating element whose upper end is designed as a knifelike, sharp severing edge and whose inner wall surface facing toward the central region of the curtain being poured constitutes an uninterrupted, smooth face extending in a single plane from the said severing edge to a lower foot-end edge at the foot end of the separating element, while the inclination of the aforesaid inner wall surface relative to the perpendicular is adjustable in a manner such that the trimmed central region of the poured curtain flows downward with laminar flow and with lateral spreading along the aforesaid inner wall surface and is severed from that surface at the said lower foot-end edge, whereby it can impinge with almost the same lateral spreading onto a substrate to be coated in a horizontal coating plane extending transversely to the poured curtain, thereby depositing a coating layer, any marginal bead of which layer being formed therein is narrow and is at most twice as high above the coating plane as the bead-free regions of the still undried layer.
A coating is "undried" when freshly deposited on a substrate by the curtain and still containing a major portion of the solvent present in the solution mentioned hereinbefore as coating material. The width and height of marginal beads occurring in coated zones on a substrate can be reduced more when using separating elements according to the invention bearing one or preferably several of the following inventive features:
The smooth inner wall surface of the separating element is of shorter length, and the lower foot-end edge thereof hence less distant from the severing edge thereof, than if the separating element were of such length that the central curtain region, flowing downward with the spreading portion thereof remaining laterally in contact with said inner wall surface due to adhesive forces acting between the material of the curtain, on the one hand, and that of the inner wall surface, on the other hand would become detached from the latter only as the inherent weight of the adhering curtain overcomes the said adhesive forces.
The inclination of the inner wall surface of the separating element relative to the perpendicular is adjusted in a manner such that the lower foot-end edge is at a distance of about 3 to 8 mm from the perpendicular, which distance is measured in a plane extending through the said foot-end edge parallel with the coating plane mentioned further above.
The inclination of the smooth inner wall surface relative the perpendicular amounts to about 3.degree. to 25.degree., and preferably to from 5.5.degree. to 15.degree..
The length of the smooth inner wall surface from the upper sharp severing edge to the lower foot-end edge amounts to about 10 to 500 mm and preferably from 30 to 70 mm.
The upper severing edge is rendered knifelike, sharp by means of an oblique face provided at the upper end of that wall surface of the separating element facing away from the curtain central region, which oblique face encloses with the smooth inner wall surface of the separating element an acute angle.
The smooth inner wall surface of the separating element can be plated with a noble metal such as silver, gold or a metal of the platinum group in order to enhance adhesive forces with the coating material.